The present invention relates to improvements in an internal combustion engine, especially in an internal combustion engine for automobiles. As is well-known in the art, in conventional gasoline engines for automobiles in which an air fuel mixture that is somewhat richer than a theoretical air-to-fuel ratio is burnt within a cylinder, a large amount of harmful gases, that is, NO.sub.X, CO and HC, are contained in the exhaust gas. Among such harmful gases in the exhaust gas, HC and CO can be converted into harmless materials by burning them within a thermal reactor provided, for example, within an exhaust system or by oxidizing them with a catalytic converter. However, to this end, additional air serving as an oxygen source and normally called the secondary air must be supplied to the exhaust system, so that an air pump or another air source is required, resulting in an increase in the manufacturing cost of engines.
On the other hand, it has been known that gasoline engines have the following three characteristic performances:
1. As the air-to-fuel ratio of the gaseous mixture of air and fuel becomes excessively richer or excessively leaner than the theoritical air-to-fuel ratio, the NO.sub.x concentration in the exhaust gas produced by burning of the gaseous mixture becomes lower.
2. When a gaseous mixture, the air-to-fuel ratio of which is richer than the theoretical mixing ratio is burnt, generally the concentrations of CO and HC in the exhaust gas are high.
3. When a gaseous mixture, the air-to-fuel ratio of which is leaner than the theoretical mixing ratio is burnt, concentrations of CO and HC are low, if misfire does not occur.
(The term "air-to-fuel ratio" as used herein means the ratio of the amount of air (weight)/amount of fuel (weight) in the gaseous mixture.)
In the light of the above-mentioned three points, a multi-cylinder internal combustion engine in which an excessively lean gaseous mixture is fed to half of the cylinders but an excessively rich gaseous mixture is fed to the remaining half of the cylinders, has been already proposed. In this type of internal combustion engine, from the cylinders in which a lean mixture is burnt, there is discharged an exhaust gas which contains a relatively small amount of NO.sub.x and a small amount of CO and HC, but which contains a large amount of excess oxygen, whereas from the cylinders in which a rich mixture is burnt, there is discharged an exhaust gas which contains a relatively small amount of NO.sub.x, a large amount of CO and HC and a small amount of excess oxygen.
Therefore, by mixing the exhaust gas discharged from the cylinders to which said rich gaseous mixture is fed and the exhaust gas discharged from the cylinders to which said lean gaseous mixture is fed within an exhaust system, the oxygen necessary for combustion of the CO and HC discharged mainly from the former cylinders can be supplied by the exhaust gas discharged from the letter cylinders, so that in an ideal case, feeding of secondary air is not necessary at all, and even if feeding of secondary air is necessary, the amount can be less than in the case of the conventional engines. Accordingly, there exists the advantage that an air pump or other air source serving as a secondary air source can be omitted or its capacity can be made extremely small.
However, with regard to the method for producing an excessively rich gaseous mixture in such type of internal combustion engine which has been proposed in the past as described above, since the general method for producing a gaseous mixture was by making use of a throttle valve and a negative suction pressure in a carburetor as heretofore employed in engines in general and by merely changing the cross-sectional area of the air suction pipe and the diameter of the fuel feed jet opening, the method had a disadvantage that the adjustment of the relative amounts of air and fuel in a lean mixture could not be maintained at an optimum over the entire operation range of an engine. More particularly, there were various disadvantages such as an increase of production of NO.sub.x from the cylinders to which a rich gaseous mixture is fed, and reduction of powder output from the cylinders to which a lean gaseous mixture is fed, both due to dilution of the gaseous mixture upon acceleration; increase of production of HC and CO from the cylinders to which a rich gaseous mixture is fed, and generation of after-burn both due to the concentration of the mixture upon deceleration; and excessive dilution of the lean gaseous mixture fed to all the cylinders, especially causing misfiring in the cylinders to which only a lean mixture gas is fed.